1. Field of the Invention
This invention relates to a soldering flux adaptable to several different types of electronic soldering applications, including microelectronic applications. More particularly, the flux of this invention contains borneol, an agent that acts to reduce residues and/or increase allowable activator concentration, and to impart tackiness to the flux when used in larger proportions. The soldering applications include the soldering of microelectronic chip components to metal films on composite material circuit boards and to cermet conductor films on ceramic substrates, and the reflowing of solder on contact bumps of semiconductor wafers.
2. Description of the Prior Art
Commercially used fluxes are often tailored in composition for given soldering applications, depending on the needs of those particular applications. For example, active and passive microelectronic chip components are wave soldered to a metal pattern on a composite circuit board. Such chip components can be resistors, capacitors, and integrated circuits. By composite circuit board, we mean a paper-reinforced epoxy or phenolic supporting substrate material having a foil and/or plated metal, usually copper, pattern thereon. In wave soldering chip components to such circuit boards, the chip components are usually temporarily bonded to the metal pattern side of the circuit board with a small amount of binder. The circuit board is then inverted for wave soldering, so that the metal pattern and the chip components are then disposed on the underside of the board. Typically, flux is applied to the underside of the board by horizontally moving the circuit board over a standing wave, or a spray, of flux. Next, the fluxed board is horizontally moved over a standing wave of molten solder. The flux preferably wets all surfaces it contacts, including the chip binder as well as conductor surfaces, to insure that the flux reaches all the parts of the chip electrodes and metal pattern where soldering is desired. If the flux has significant tackiness, it can help retain the chips affixed to the circuit board during the thermal shock and fluid flow of the solder wave. If the metal is relatively clean to start with, the oxide-cleaning requirements of the flux may not be particularly demanding. On the other hand, in many applications, flux residue, and its cleanability can be an important issue, as will hereinafter be discussed.
The soldering of active and passive microelectronic chip components to ceramic substrates have their own distinctive soldering issues to consider. Hybrid integrated circuit modules are made from such substrates. Microelectronic chip components are soldered to a cermet conductor pattern on the ceramic substrate, while the substrate is face up. The demand for tackiness in this application is significantly less. However, the oxide-cleaning demands are significantly higher. Hence, the flux may have to include a higher proportion of activator, which can lead to higher ionic residues from the flux after soldering. Moreover, in these applications wettability and cleanability are usually quite important. In addition, it is usually preferred that our flux leave little or no organic residue. Flux residue, if any, should be readily cleanable at the very least. However, it is most preferred that the flux leave little or no ionic or organic residue that has to be removed by cleaning.
Still another soldering application involves a semiconductor wafer having contact bumps thereon for each integrated circuit chip incorporated in the wafer. The wafer, such as of silicon, has a dielectric coating of silicon nitride or the like on its surface with a plurality of contact windows in the dielectric coating for each integrated circuit chip incorporated in the wafer. A contact bump is disposed in each window. Solder is deposited on each contact bump. The bump solder is melted at least once and preferably twice, to re-flow the solder to obtain a uniform bump height, geometry and appearance. The wafer is fluxed during such re-flow. In such an application, it is important that the flux used in this particular application have good wettability on the dielectric surface, good spinability for application to the dielectric surface, and have good cleanability. As to cleanability, it is much preferred if the flux leaves little or no ionic or organic residue after solder reflow, so that the wafer surface does not have to be cleaned at all.
In recent years, commercial interest in "no-clean" and low residue fluxes has significantly increased. The interest has moved from the simple desire to leave no visible residue to the unaided eye, to actually measuring the extent of ionic residue, even if it cannot be seen with the unaided eye. Initially, "no-clean" fluxes simply did not require post-soldering cleaning with environmentally unfriendly cleaners, as for example chlorofluorocarbon solvents or chlorinated solvents, which rosin-based fluxes normally required. However, such "no-clean" fluxes may or may not have left a residue that was visible or not to the unaided eye. However, if any residue was left, it could be readily removed by rinsing with water or some other environmentally friendly solvent. Most recently, cleanliness requirements have increased. A truly low residue flux, by current standards, leaves little or no measurable ionic or organic residue, even when no residue can be seen with the unaided eye. By way of example, truly low residue fluxes are described in each of U.S. Pat. No. 5,004,509 Bristol and U.S. Pat. No. 5,507,882 Bristol, Hanaway and Walls, and also described in the U.S. patent application Ser. No. 08/559,543, filed Jul. 11, 1995 in the names of S. V. Bristol, E. H. Hanaway, T. R. Mueller, and M. J. Barnaby. The teaching of theses prior patents and applications are incorporated herein by reference.
For soldering chip type components to circuit boards, it is also known that camphor provides enhanced tackiness benefits to fluxes during soldering. As indicated above, the tackiness helps retain the electronic chip components in place during soldering operations.